Balanced amplifiers are widely used in the amplification of radio frequency (RF) signals due to their exceptional performance in many practical situations. Specifically, balanced amplifiers often exhibit, good input and output return losses, and better stability when compared to single-ended amplifiers. An exemplary conventional balanced amplifier 10 is shown in FIG. 1. The conventional balanced amplifier 10 includes an RF input node 12, an RF output node 14, an input termination impedance 16, an output termination impedance 18, a first amplifying device 20, a second amplifying device 22, an input quadrature coupler 24, and an output quadrature coupler 26. The input quadrature coupler 24 includes a first input node 28 coupled to the input termination impedance 16, a second input node 30 coupled to the RF input node 12, a first output node 32 coupled to an input of the first amplifying device 20, and a second output node 34 coupled to an input of the second amplifying device 22. The output quadrature coupler 26 includes a first input node 36 coupled to an output of the first amplifying device 20, a second input node 38 coupled to an output of the second amplifying device 22, a first output node 40 coupled to the RF output node 14, and a second output node 42 coupled to the output termination impedance 18.
In operation, the conventional balanced amplifier 10 is configured to receive an RF input signal RF_IN at the RF input node 12 and produce an amplified RF output signal RF_OUT at the RF output node 14. Specifically, the conventional balanced amplifier 10 is configured to receive an RF input signal RF_IN with a phase angle of zero degrees at the RF input node 12. As the RF input signal RF_IN enters the input quadrature coupler 24, the signal is split into an in-phase portion and a quadrature portion. The in-phase portion of the RF input signal RF_IN is equal to the RF input signal RF_IN over the square root of two (0.707 multiplied by the RF input signal RF_IN) at a phase angle of zero degrees, while the quadrature portion of the RF input signal RF_IN is equal to the RF input signal RF_IN over the square root of two (0.707 multiplied by the RF input signal RF_IN) at a phase angle of −90 degrees. The in-phase portion of the RF input signal RF_IN is delivered to and amplified by the second amplifying device 22, while the quadrature portion of the RF input signal RF_IN is delivered to and amplified by the first amplifying device 20. The resulting amplified in-phase portion of the RF input signal RF_IN is delivered to the second input node 38 of the output quadrature coupler 26, while the resulting amplified quadrature portion of the RF input signal RF_IN is delivered to the first input node 36 of the output quadrature coupler 26.
The output quadrature coupler 26 shifts the amplified in-phase portion of the RF input signal RF_IN at the second input node 38 by −90 degrees and delivers both the amplified and phase-shifted in-phase portion of the RF input signal RF_IN and the amplified quadrature portion of the RF input signal RF_IN (with an unchanged phase) to the RF output node 14. Accordingly, the amplified and phase-shifted in-phase portion of the RF input signal RF_IN and the amplified quadrature portion of the RF input signal RF_IN each have a phase equal to −90 degrees, and therefore combine to produce an RF output signal RF_OUT equal to the gain of the amplifying devices multiplied by the RF input signal RF_IN at a phase angle of −90 degrees. Further, the quadrature output coupler 28 shifts the quadrature portion of the RF input signal RF_IN by −90 degrees and delivers both the amplified and phase-shifted quadrature portion of the RF input signal RF_IN and the amplified in-phase portion of the RF input signal RF_IN (with an unchanged phase) to the output termination impedance 18. Since the amplified and phase-shifted quadrature portion of the RF input signal RF_IN and the amplified in-phase portion of the RF input signal RF_IN are of equal magnitude and are also 180 degrees out of phase with one another, these signals effectively cancel.
As the load provided at the RF output node 14 changes to become mismatched with the output termination impedance 18, for example, due to a change in the impedance of an antenna connected to the RF output node 14, the balanced amplifier experiences what is known as “load pull” due to a high voltage standing wave ratio (VSWR). Specifically, the magnitude of the amplified in-phase portion of the RF input signal RF_IN and the amplified quadrature portion of the RF input signal RF_IN become mismatched, and therefore the signals no longer cancel at the output termination impedance 18 as discussed above. This results in a buildup of voltage across the output termination impedance 18, which may eventually result in damage to the output termination impedance 18 as well as damage to the first amplifying device 20 and/or second amplifying device 22. Further, this results in thermal stress on the first amplifying device 20 and/or the second amplifying device 22, reduced efficiency, and higher voltage swings at the device terminals.
In an effort to protect the conventional balanced amplifier 10 from damage due to high VSWR conditions, external isolators have been used in conjunction with the output termination impedance 18. FIG. 2 shows the conventional balanced amplifier 10 including an external isolator 44 coupled in series with an additional output termination impedance 45 between the RF output node 14 and ground. The external isolator 44 may be a circulator, which may consume a large amount of area and further add expense to the surrounding circuitry of the conventional balanced amplifier 10. Further, the external isolator 44 may degrade the efficiency of the conventional balanced amplifier 10. As shown in FIG. 2, the conventional balanced amplifier 10 may be integrated onto a semiconductor die, represented by the dashed box 46 shown in FIG. 2. However, the external isolator 44 cannot be integrated onto the semiconductor die 46 due to the size thereof. Accordingly, there is a need for a balanced amplifier that is capable of safely dealing with high VSWR conditions while simultaneously being efficient and compact.